1. Field of the Invention
This invention relates to a heat exchanger for gases at high temperature, in particular to the heat transfer of a high temperature reactor from a primary gas circuit to a secondary gas circuit, with the secondary gas conducted through multiple, parallel connected U-tubes in counterflow to the primary gas.
2. Description of the Prior Art
Heat exchangers having heat transfer surfaces consisting of U-tubes have considerable advantages compared to straight tubes, particularly as evaporators, because the U-tubes are fixedly connected at both of their ends, but can freely expand with their U-bends with respect to the housing, or with respect to their suspension. U-tube heat exchangers also have important advantages compared to heat exchangers with helical tubes which have been proposed for high temperature gases. They are easier to manufacture, and also easier to install and, therefore, more advantageous pricewise. U-tubes are easier to check after the installation and also after more extended operating time. They also can be repaired with greater ease, because one can examine and test the long, straight legs of the U-tubes from inside readily and reliably by means of long probes, a task that is very difficult with heat exchangers with helical tubes because of their complicated shape. Furthermore, a gas-heat exchanger which is operated using counter-flow has only a small, and over the length of the tube approximately constant temperature difference between the primary and the secondary medium, so that temperature differences of a magnitude large enough to cause impermissible tension cannot occur in the tubes themselves, nor in their suspensions, nor in the channel walls surrounding the tubes. Notwithstanding these advantages, there are, however, considerable problems when using a U-tube heat exchanger for gases at 950.degree. C. For example, with such hot gases one must separate the cold gas lead-in line from the heated gas lead-out line. The corresponding cold and hot gas collectors must be different spacewise and constructionwise, to avoid stresses (tension) between construction members at different temperatures, and also to avoid undesirable heat losses. The gas input, the gas output lines and the corresponding collectors for the cold and hot gas are of large dimensions, at varying operating conditions cause very different expansions, particularly in the longitudinal direction. The U-tubes themselves cannot absorb these expansions because, at the high temperatures, the additional stresses still permissible for the materials used, are low.